Answer from cs61c-ev (Roland Carlos 16583196) for Question 3
Xs in truth tables represent don't cares, which mean that their value does not matter. In the case of this table, where the x's are in the inputs, the xed out inputs do not matter when determining output. So, for the first line, if a is equal to zero, then regardless of b or c's values, the output will be 1.

not(a) + (a . not(c))

The table is complete. It represents all inputs, although some of them are not shown because of the don't cares.